The invention relates to a process for the preparation of derivatives of dichloroacetic acid esters by reaction of the corresponding glyoxylic acid esters with phosphorus pentachloride.
Such a reaction has in the past been effected by allowing solid powdery phosphorus pentachloride to act on the corresponding glyoxylic acid ester (see Bull. soc. chim. de France 1959, pages 850-853). The desired product is then obtained at a yield of only the order of 45-55% of the theoretically possible yield.
The present invention provides a novel process by which the said reaction with phosphorus pentachloride can be effected to obtain a substantially higher yield.
The novel process according to this invention is for the preparation of derivatives of dichloroacetic acid ester, by reaction of the corresponding glyoxylic acid esters with phosphorus pentachloride, and is characterized in that a glyoxylic acid ester having the general formula R.sub.1 --CO--COOR.sub.2, wherein R.sub.1 represents an optionally substituted aryl or heteroaryl group and R.sub.2 is an alkyl, cycloalkyl or aryl group, is reacted with the phosphorus pentachloride at a temperature of 35.degree.-150.degree. C., while in the presence of a liquid dispersant, suitable for distributing the phosphorus pentachloride through the reaction mixture, with the formation of the desired corresponding dichloro derivatives having the general formula R.sub.1 --CCl.sub.2 --COOR.sub.2, wherein R.sub.1 and R.sub.2 have the above definitions.
By employing the process according to this invention the desired product can also be recovered from the reaction mixture at a sufficient level of purity such that the product recovered can be used for further applications without itself having to be distilled.
The reaction with phosphorus pentachloride according to this invention is exothermic. Therefore, in order to maintain the desired temperature, effective means for the removal of the heat of reaction must be employed. The desired reaction temperature is preferably maintained by using a liquid distributor having a boiling point corresponding with the desired reaction temperature. The heat of reaction can then be carried off by evaporation of the dispersant itself. The quantity of liquid dispersant present in the reaction mixture can be kept constant through application of suitable reflux condensation.
The reaction temperature may accordingly be varied within the above-stated limits; however, it is advantageous to maintain the temperature between 70.degree. and 100.degree. C.
In practicing the process according to this invention various dispersants suitable for distributing phosphorus pentachloride may be used, for instance alkanes or mixtures of alkanes, such as n-heptane and n-octane, cycloalkanes or mixtures of cycloalkanes, such as cyclopentane and cyclohexane, aromatics or mixtures of aromatics, such as benzene, toluene and xylenes and chlorinated hydrocarbons or mixtures of chlorinated hydrocarbons, such as chloroform, 1,2-dichloroethane and tetrachlorocarbon. Inorganic dispersants may also be employed such as, for instance, phosphorusoxytrichloride. The precise amount of the dispersant employed is not critical provided it is in sufficient quantity to effect complete distribution of the phosphorus pentachloride through the reaction mixture.
Various starting glyoxylic acid ester materials may be used. For instance, suitable compounds include those in which the group R.sub.1 in the above general formula represents a phenyl, naphthyl, pyridyl, furyl or thienyl group, which groups may optionally be substituted with one or more substituents, e.g., of the class of Cl, NO.sub.2, alkoxy with 1-8 C atoms and alkyl with 1-8 C atoms. As the R.sub.2 group in these starting compounds an alkyl group with 1-8 C atoms, a cycloalkyl group with 5-8 C atoms in the ring, a phenyl group or a naphthyl group, for instance, may be used. These starting compounds can be very effectively prepared by the procedure described in copending U.S. application Ser. No. 320,562, filed Nov. 12, 1981 the disclosure of which is incorporated herein by reference.
The reaction mixture obtained according to this invention can, for instance, be worked up by distilling off the dispersant and the phosphorusoxytrichloride formed in the reaction, subsequently converting with water any phosphorus pentachloride still present into phosphoric acid and hydrogen chloride, separating the organic layer formed from the aqueous layer, washing the organic layer with water and, if desired, further purifying it by distillation. If in this working-up process the dispersant and the phosphorusoxytrichloride formed in the reaction process are not distilled off, an organic layer can be obtained which still contains the dispersant and which can be used as such for further conversions. The reaction mixture can be worked up also by distilling off the dispersant and phosphorusoxytrichloride formed and incorporating the still residue in an organic solvent from which any residual phosphorus pentachloride precipitates such as, for instance, cyclohexane. After separation removal of the phosphorus pentachloride an organic solution will be obtained from which the product obtained can be recovered by distillation if so desired.